Multi-cell, rechargeable batteries are used in a variety of industrial and commercial applications such as fork lifts, golf carts, uninterruptable power supplies, and electric vehicles.
Rechargeable lead-acid batteries are a useful power source for starter motors for internal combustion engines. However, their low energy density (about 30 Wh/kg) and their inability to reject heat adequately, makes them an impractical power source for an electric vehicles (EV), hybrid electric vehicles (HEV) and 2–3 wheel scooters/motorcycles. Electric vehicles using lead-acid batteries have a short range before requiring recharge, require about 6 to 12 hours to recharge and contain toxic materials. In addition, electric vehicles using lead-acid batteries have sluggish acceleration, poor tolerance to deep discharge, and a battery lifetime of only about 20,000 miles.
Nickel-metal hydride batteries (“Ni—MH batteries”) are superior to lead-acid batteries and are the ideal battery available for electric vehicles, hybrid vehicles and other forms of vehicular propulsion. For example, Ni—MH batteries, such as those described in U.S. Pat. No. 5,277,999, the disclosure of which is incorporated herein by reference, have a much higher energy density than lead-acid batteries, can power an electric vehicle over 250 miles before requiring recharge, can be recharged in 15 minutes, and contain no toxic materials.
Extensive research has been conducted in the past into improving the electrochemical aspects of the power and charge capacity of nickel-metal hydride batteries. This is discussed in detail in U.S. Pat. Nos. 5,096,667, 5,104,617, 5,238,756 and 5,277,999, the contents of which are all incorporated by reference herein.
Multi-cell nickel-metal hydride batteries may be packaged in a variety of configurations. For example, individual cells may simply be secured together with the use of end plates and a strap to form a “bundle” of individual cells. Alternatively, the individual cells may be all be housed within a common outer battery case. Examples of multi-cell batteries are provided in U.S. patent application Ser. No. 09/139,384, now U.S. Pat. No. 6,255,015 the disclosure of which is incorporated herein by reference.
The electrochemical cells of multi-cell batteries may be electrically coupled in series by conductive links, or they may be formed in a bipolar configuration where an electrically conductive bipolar layer serves as the electrical interconnection between adjacent cells as well as a partition between the cells. To be successfully utilized, the bipolar layer must be sufficiently conductive to transmit current from cell to cell, chemically stable in the cell's environment, capable of making and maintaining good electrical contact to the electrodes and capable of being electrically insulated and sealable around the boundaries of the cell so as to contain electrolyte in the cell. Examples of bipolar batteries are provided in U.S. Pat. Nos. 5,393,617, 5,478,363, 5,552,243, and 5,618,641, the disclosures of which are all incorporated by reference herein.
The requirements for making high quality multi-cell rechargeable batteries become more difficult to achieve in the case of nickel-metal hydride batteries due to the charging potential of the cells which can accelerate corrosion of battery components, to the creep nature of the alkaline electrolyte that can cause self-discharge between cells, and to the higher cell pressures which can deform and damage the cell enclosures. The present invention provides an improved design for rechargeable multi-cell batteries applicable to all battery chemistries and, in particular, to the rechargeable nickel-metal hydride chemistry.